Just the whole thing of what happened with Schala and Janus is pretty much evidence that they can't be of any kind of Guardia line. However, it's not impossible to think that while it wasn't mentioned, someone in the Earthbound community may have been part of Marle's bloodline.
Now this of course is theory, but in the original timeline where Zeal fell anyway, Schala may have been able to escape there or at minimum get her pendant out. Now while it had no power left by this point, it's not impossible to think that the Earthbound ones (maybe one of Marle's ancestors) discovered the pendant and kept it as an heirloom.
Of course, this theory wouldn't necessarily explain why the pendant is not present in 600 AD if it was kept as an heirloom (or it was and just not mentioned at all in that timeline due to any possible lack of importance). We could always say that the pendant was also lost through the fall of Zeal and someone in the Guardia line discovered the pendant anywhere between 600 and 1000 AD.
Or maybe something that's very far reaching (mind you, either of those theories regarding the pendant could have happened with this), but I'm sure we remember one of the Earthbound ones saying that some of the Enlightened did survive, but they have no distinctions anymore between them. Maybe Marle's ancestor's back in this time could have been one Enlightened one and one Earthbound one....After all, this theory would support bringing humanity back into one category and while magic was pretty much gone in humanity, it would still keep the innate magic ability within many people as time progressed.
